Where Have All the Grasses Gone? Eco-Trouble in Texas Big Bend National Park

What has happened to the grasslands in Texas' Big Bend National Park? One hundred years ago, early settlers encountered an endless vista of grassland in the area that is now the park. Those grassland have now vanished.

Big Bend is one of the most remote parks in the United States. Hundreds of miles away from any major city, the park is located in the lower part of the Rio Grande River adjacent to the Mexican border.

"I had no idea things were so severe and that there was such vast acreage involved," hydrologist Carol Purchase said. Purchase works for Big Bend National Park and will present her findings about the park's conditions April 11 at the Geological Society of America's South-Central Section Meeting in Alpine, Texas.

"Grazing in the early part of the century definitely had a large effect by removing the vegetative cover on these very fragile soils resulting in high erosion rates," she said. "After the grass was gone, many of the ranchers built lots of water diversion dams--typically 100 yards long and 6 to 10 feet high, to back up the water to grow grass or to divert the water into stock ponds. These dams have really aggravated the problem by concentrating the flow and we now have large gully systems that cover almost 1,000 acres. We are afraid that the area is headed towards an eroded badlands shrub ecosystem and are not sure how much of the area we can restore to grasslands."

Purchase will try experimental treatments this summer by introducing native grasses to the soil. But the degree of damage to the soil in the area is another issue. Believing the soil had nutrient deficiencies, she asked John Zak, a biologist at Texas Tech University, to test some samples. To her surprise, the soil was actually overloaded with nitrogen. In fact, new research reveals that the nitrogen level might increase to the point that not even desert vegetation could tolerate it. Purchase is not sure how the nitrogen got there but thinks some of it could be from the air. Declining air quality is another significant problem in the park.

"It's pretty unusual to have such nitrogen levels in depleted soil," she said. "It means that the soil is pretty dead."

"It is hard to believe that this broad valley once held verdant grasslands that grew 'knee-deep to a horse,'" Purchase wrote in her abstract for the meeting. "Once, herds of pronghorn raced across these flats, desert bighorn sheep entered the valley to visit springs, and Mexican gray wolves worked the area in search of prey. Now, all are gone."